Lecture desk



Nov. 28, 1939.

D. KATZ LECTURE DESK Filed Feb. 2e, 193s :s sheets-sheet 1 INVENT'R Nov.28, 1939. D, KA-rz I 2,181,133

' LECTURE DESK Filed Feb. 26, 1938 3 Sheets-Shet 2 INVENTOR Nov. 28,1939. D. KATZ 2,181,133

LECTURE DESK Filed Feb. 26, 1938 3 Sheets-Sheet 3 A5 X f Patented No'v.28, 1.939

PATENT oFFlcE LECTURE DESK David Katz, Wilmington,v Del.

Application February 26, 1938, Serial No. 192,755

12 Claims.

This application is a continuation-in-part of `my copending applicationSerial No. 155,390,

led July 24, 1937, and relates to certain novel lecture desks which areprovided with means for displaying to the audience writing or sketchesdone by the lecturer during the course of his lecture. f

More particularly, this invention deals with a novel lecture deskwhichenables the lecturer to illustrate his discussion with writing orsketches,

and this shadow is focussed, with suitable inver.

`sion and change of direction, by an optical system within the interiorof the desk chamber, onto a projection screen placed in a substantiallyvertical position in a cut-away space in the rear y wall of the desk,that is the wall normally facing the audience. The Iprojection screen ismade of -3'0 shadow transmitting material, whereby the shadowsfocussedupon it from the inside of the` desk show through to the outer facethereof, confrontthe descriptionpmceeds.

ing the audience.

In the model described in my copending application, the writing panelconsists of a at sheet of translucent material.` 'I'he latter may beeither ground glass or a sheet of translucent paper; but in any event,the faces of the writing panel are two planes parallel to each other.'I'he 40' optical system, which consists of a convex lens and a planemirror, or a systemcf lenses and mirrors, depends for its operation onthe scattered light which reaches the lens from the various particles inthe translucent writing medium.

Now, I nd that the above system gives satisfactory results only wherethe source of illuminationhas a wide area compared to the apertureof-the lens employed, as for instance where a number of electric lampshave their light reflected uniformly unto the writing plane by the aid Aof a reector of large effective area. Such a system is practical withsmall models. With larger models, however, the required size of thereector area .tends to diminish considerably the practical adaptation oftheldevice, On the by my present invention which consists, in genotherhand, if the requisite illumination is permitted to come from arelatively small area, the projection on the screen is found to containa bright spot in the middle of the screen area, with practically noillumination, and hence no shadow 6 elects, in the'surrounding regions.

It is accordingly an object of this invention to provide an improvedlecture desk of the general type above indicated which will givesatisfactory results even with a lrelatively concentrated source 10- oflight. A further object of this invention is ,to provide an improvedarticle of the above general type wherein the usable portion of theprojection screen will be substantially uniformly illuminatedlthroughout its expanse, with resultant good 15 lightand-,shadowcontrast, regardless of the' shape or size of the illuminating meansemployed. .A still further object of this invention is to improve thedesign of my lecture desk aforesaid whereby it may be made to givesatisfac- 20 tory results with less strong illumination than isvrequired with my prior model, resulting not only in-a saving ofelectric power but also in greater comfort to the lecturer in that helneed not be exposed to the radiant heat from a large 25y cluster oflamps. Y

A- still further object is to provide a design which lends itself morereadily to modiiication for the production of large size models, thanwas my former design. Other and further im- 30 portant objects of thisinvention will appear as These and other useful objects are achievederal terms, of replacing the heretofore employed 3g doubly plane writingpanel by an optical member which is adapted to combine the functions ofa writing plane with those of a light-condensl ing lens. In other words,the cut-away opening in the top of the desk, instead of being lled by apanel of glass which is plane on both sides, is according to my. presentinvention, lled by a member of glassy material which is flat on theyupper surface thereof. and serves there as a writ` ing plane, but iseither shaped like a convex lens on the lower side or'else combined witha condensing lens of any suitable form, wherebyv to bend light rayspassing through it, into the general direction of the optical systembelow. t

For a better understanding of my present invention, reference isto bemade to the accoml panying drawings which; form an integralpart deskbuilt according to this invention, and employing a plano-convex lens aswriting panel.

Fig. 2 represents a similar section through the foremost top portion'oithe desk in a modiied form of this invention, employing an annularFresnel lens.

Fig. 3 is a 'vertical section of a still further modied embodiment of myinvention.

Fig. i is a perspective view, partly in section, of the lens memberemployed -as writing panel in the desk of Fig. 1, on a slightly enlargedscale.

Fig. 5 is an enlarged sectional view of the annular Fresnel lensemployed as writing panel in- Fig. 2.

Fig. 6 is an optical diagram, showing 'in simplified fashion one mode ofdisposition of the illuminating'` means, writing-panel lens, focussinglens ard projection screen relative to each other.

Fig. 'l is a similar diagram showing a different mode of relativedisposition.

Fig. 8 is a similar diagram showing a still different mode ofdisposition.

Fig. 9 is a similarv diagram showing a modied form of disposing theilluminating means in any of the aforegoing'designs.

Fig. 10 is a simpliied optical diagram showing the focussing of thepoints of shadow which come from the writing plane unto the projectionscreen.

'I'othe weight (or spring) 62 over the roll 6I at the ing the interior.The inner partition has for the same reason been partly broken away.

Fig.. 12 is a diagrammatic pictorialization of a K modied form of theoptical system to be employed in commotion with the embodiment displayedin Fig. 11.

Fig. 13, is a similar, three-dimensional diagramof a still furthermodied form of the optical system. e

Fig. 14 .is an enlarged perspective view of the tilted-mirror systememployed in Fig. 11.

Fig. 15 is a section either through line X-X or through line Y-Y in thewriting panel of F18. 11. l

Referring now to Fig. 1 in detaili, l constitute the walls,.t op andfloor of the lecture desk which are made of opaque desk material, andare joined together to form an enclosed darkchamber, as in my formerapplication. 2 is the writing panel, Awhich will be described in detaillater. 3 is the projection screen, made of shadow transmitting materialand supported pivotally at 3|, with/means for adjustment at 32. I is thefocussing lens and 5 is the mirror of the optical system, bothbeing-supported lon a suitable support 'l which is adjustable inposition through screw 1I and slit 'l2 in the wall of the desk; nneradjustments of the lens being obtainable by the aid of the threaded tube42 for the lens, and axis 5| and set screw 52 for the mirror. I

Under the eave of the desk on the side neares the lecturer (or at anyother convenient location) is a `ro1l of light-transmitting paper 6I,from which a sheet 6 stretches over the surfaceof the writing plane 2,and is held in tension by the pivoted knife edge device Biron one endand by other ends. The desk is further provided on the from beingscattered unto the projection screenl at the bottom thereof to permitthe principal' beam of light to pass into 'the lens 4.

So far the novel lecture desk is substantially identical with the deskdescribed in Fig. 2 of my copending application, Serial No. 155,390. The

are not inventive differences, being within the scope of my originalinvention above referred to, But the principal difference between thedesk of Fig. 1 of this application and the one of my copendingapplication is in the nature of the writing panel 2.

Instead of being a flat pane of glass as in my previous design, thewriting panel in Fig. 1, here, is a plano-convex lens. particularly theupper surface, is at, as shown at 26 in Fig'. 4, and forms the writingplane. The other surface, however, is convex, as at 2l, and is ofsuicient curvature to bend the light rays coming from the illuminatingmeans i5 into the general direction of the lens'. Lenses of this typeare knownsgenerally as condensing lenses.

Precise focussing is not needed, and the location of the focus may beeither in advane of or beyond the lens The aperture of this lens, thatis its width One surface of it, more from edge to' edge, may be anywherefrom about 6 to 24 inches, depending on the size of writing planedesired. Its outlinemay be circular, or lit may be vsquared o i, as at2l, 2| in Fig. 4. The material of the plano-convex writing panel may beglass or any suitable, hard, transparent material, for instance Lucite(a methyl-methacrylate resin) Or it may be compounded from two diiferentmaterials, for instance glass and Lucite, crown glass and iiint glass,as is customary in lens design. But the thickness of the glass should bekept .down to a minimum, inasmuch as a thick lens is apt to distort theshadows on the projection screen. For this reason, the radius ofcurvature, and consequently the focal length, of the condensing lensshould be as large as possible. A

The writing-panel lens may be lightly ground on its at surface, to bewritten on directly with a soft pencil or crayon. But it isl preferableto have this lens clear throughout, and to use a sheet of transparentpaper as the actual writing surface. On th other hand, extreme opticalperfection,such as freedom from aberration, is

not essential to its performance in the present invention.

The lens is mounted in the top of the desk so that its plane face 26issflush with the top lsurface of the desk, to provide a continuous flatsupport for the writing paper. Since the width of thelens is foreconomical reasons kept down to a minimum, the writing is preferablydone with a lead pencil or sharpened crayon or ink, so that, the guresand letters are relatively small. 'I'he optical system within the desk,and projection screen can be designed to effect any desirable degree of,enlargement.

Cooperating with the plano-convex lensis a source of illuminationi l5,xed at a definite distance above the writing` plane by means of anadjustable support I I. 'I'his support may be independent of the desk ormay be attached thereto, but to save .the necessity of delicateadjustments in installation, the lamp support is preferably designed andbuilt as a part of the desk. 'Ihe lamp l5 is provided with asuitablereilector the partition l is provided with an opening Il |0,preferably bearing-.at its end a condensing u is shown diagrammaticallyin Figs. 6 to 9, in-

clusive, wherein the plane mirror 5 has been omitted for simplicity,inasmuch as it does not ailect the principles of this discussion.Omissionl of the mirror enables us to represent the light raystravelling fromthe writing plane to the screen by straight lines,although in actual practice they are broken lines. fore that in actualpractice the distance between the focussing lens 4 and the projectionscreen 3 is to be measured along the broken paths of the light raysinstead of along the shortest path between the two. Flgs.6 to 9,together with Fig. 10, also bring out the quantitative relations involvedl and aid one in selecting the proper dimensions and design datafor any particular A purpose.

In Figs. 6 to 10 inclusive, W represents the writing plane, orcondensing lens used as writing panel, as well as the diameter thereof.L represents the shadow focussinglens, kas well as the diameter thereof.S represents thel projection screen as Well as the diameterthereof,assuming that the screen will be Acircular and just of the rightdiameter to accommodate the image 'pro- .iected from the field of W. Incase W.and S are rectangular or square, -these letters may be consideredas representing the length of their respective diagonals. O representsthe source o'f illumination, for instance the lamp I5 in Fig. lfor anarc light.4 It will be further assumed in these gures that f is thefocal length of Lwhile F is l beyond L as in Fig. 8. Regardless wherethis the focal length of W.

Now, considering rst Fig. 10, it will be observed that each opaque pointin the writing plane W emanates a beam of shadow which is condensed bythe focussing lens L and focussed upon the screen S as though it were aray of light.

This focussing effect, and the position and size of the image, areutterly independent of the position or strength of the light sourceO,and are also independent of the focal length of the lens W.Consequently, the relations shown in Fig. l hold true with any of themodifications, as to mode of condensing the illumination, shown' inFigs. 6 to 9,

inclusive.

If S is of the correct dimensions to just accommodate the image oftheentire field ofvW, then S P y From these formulas the requisitedimensions of -ables are assumed or determined by the practicalrequirements.

Referring now to Figs. 6 to 8, the focal length F of the writing panel Wmay be so chosen, and the light O may be located at such a distance,that the rays of light therefrom, after being'bent by the lens W, cometo a focus at a point in advance of L, as in Fig. 6; in the center of L,as in Fig. 7.; or

point of focus is, and regardless indeed whether there is a sharp focusat all or merely a rough condensation of the light within a smallregion,

It is to be remembered therep and q in turn are related to the focallength of .lens L by the well known lens formula all the light that haspassed throughv W will be just spread out over the area S (as definedbyFormula 1), without loss, provided L is of sucient width to interceptall these rays. 'I'his condition will be realized if in Fig. 8.

Itis clear that any value forl L which lies between those obtainable byFormulas (3) and (4)v will be satisfactory. In all these cases S will beintensely and uniformly illuminated, since all the light from W isspread out upon S, without loss.

However, if the distances q and a: be assumed as fixed, the layout ofFig. 6 will require W to have a shorter focus than in Figs. 7 and '8.The

ldesign of, Fig. 8 will give the maximum focal in 6, and

inFig.8. l l

The relation between y and a, in eithercase, is

- determined by the focal length F of the condensing lens W, by thesimple lens formula:

This one when combined with and (6) gives:

in Fig. 8. VThese formulas help select the proper dimensions for thevarious variables involved in the design of the desk, as more fullyillustrated by the following discussion.

In practice, q will be limited by the height of thelecture desk to avalue between about 2 to 3 feet, at the maximum. F cannot be greaterthany, as determined by Formula (6). If Fis to be equal to y, a: in Formula(7) must be infinite. In other words, the writing plane should beilluminated by parallel rays of light. Formula (9) then becomesconverted into 1 f 1 L f gx-W) (1) On the other hand, it is notpractical to place the light too close to the writing plane, and, as

"already mentioned, it is not desirable to have a small F value forthelens W. Both of4 these factors suggest setting a lower limit to thevalue of for instance,-a value equal to that of y. In that event, Fisjust abouti@ y. The most practical values of a: will fall somewherebetween these two extremes, which will locate the value of F somewherebetween 1/2 yand y.

To make F as near as'possible to its upper limit,

' lens C increases the value of a without bodily removing the lamp to animpractical distance. This modification also gives a sharper definitionof image and a greater contrast between writing and background,rendering the projection more easily readable from a distance, even indaylight. By placing Ov at the focus of C, the emergent light rays maybe made actually parallel; (.r= However, strictly parallel rays wouldrequireC to be of the same aperture as W, which is again objectionablefrom the viewpointof cost. Therei lfore, in practice, some divergence ispermitted,

and a: may be taken as having a value somewhere between y and 3y. Aspecic illustration will make the discussion clearer..

Assume that q is to be 30 inches, and that the aperture of L is to beabout V3 that of W. Then, by Formula (10), the maximum operative valueof F is 45 inches. This is also the value of y in Formula (6). If :c betaken' as2y, =90 inches. Then, by Formula (9), the practical value of Fis 30 inches.

The radius of curvature of a plano-convex-lens is related to its focusby the formula 'Reel-151e` (11) wherein n is the index of refraction.Assuming a material for W whose index of refraction is 1.7

(flint glass), Formula (11) becomes R=0.7 F, whence, in our problem,R=211 inches.

Assuming now that it is not desirable to have' the lens W'more thanZinches-thick at its maxi' mum, the'permissible aperture of `W iscalculated readily by plane geometry. 'I'he formula is where tV is themaximum thickness of the,lens, and R is the radius of curvature.Applying this formula, W=about 18 inches..

Now, suppose that a magnification of 2% is de-l sirable in theprojection;` then thev required diameter of.S is 21/2X18=45 inches. Ofcourse, the actual screen may be larger than the found value, and may berectangular, square, or any .other shape, provided itv accommodates thede- L. In practice, a stock size lens, approximating the determinedfocal length, will be selected for L; and the value ofp will then beadjusted to suit. In the instant case, Formula (2) suggests a lens ofabout 21 inches Ifocal length, which in turn reduces the value o f p, bythe same formula, to

70 inches. Its aperture, from the previous assumption.

is. to be about 6 inches. Here again. a larger l width does no harm.Hence a 7 or 8 inch lens may be selected if desired.

In the same problem, if athreefold magnication is desired, a lens of22.5 focus is required for L, and the screen should be not less than 54"on a side.

'I'he value of was assumed to be 90". A practical height for theilluminating means would be about ft. above`.`the writing plane, whichwould make it suiciently high to clear by a comfortable margin the headof a tall person standing Bm a platform in front of the desk. Then, byplane geometry, the aperture of thelens C must be about 6 inches. I

Of course, in lieu of a single element of light with a flash-light typelens in front of it, a fixture of f considerable dimensions, employingseveral lamps may beused. In that case the impinging light rays may bemade substantially parallel, with the result that F may be used to itsfull 45 incheswln that event, R is about 31.5"; permissible W, about22,'when t is 2, and about- 24.5" when t=2.5,

In the same problem, if crown glass or Lucite" tained only by increasingthe thickness to 3 inches, which still is fairly reasonable. On theother hand if a 2-inch thickness is insisted on, W must be reduced toabout inches. A maximum thickness of 1% inches with crown glas-s willgive a permissible aperture of about 13 inches.

In practice, it may sometimes be desirable to select a stock`lens for W,and determine q and the other dimensions accordingly. For instance, acrown-glass, plano-convexcondensing lens of 14" aperture and 21" focusis carried in stock by some manufacturers at this date. If it is desiredto build a lecture desk according to this invention from this lens, y isrst determined from Formula (7) Assuming that the lamp is placed at adistance of 5 feet and uses a condensing lens of-5" aperture, :c isabout 93 inches. 'I'hen y is about 27". Assuming now an aperture for Lof 5", Equation (6) gives q=about 17". A threefold magnification wouldthen require 21:51"r

and S=42". The requisite focal length 'of L would then be about 13".

In the same problem, if parallel light is' employed' (:c=), `theny=F=21",fI q==13.5",

p=about 40", S=about 32", and requisite f=10".

Ifin the same problem, the aperture of vvL is reduced to 4 inches, qbecomes about 19", p about 5'7", and the requisite f=14". It is clearthat all of the above designs lend themselves to practical use.

It was stated above that q is generally limited by-the height of thetable; this is due to the fact that the writing plane is substantiallyhorizontal and that the lens l is substantially vertically beneath it.However, such an arrangement is not an absolute necessity. If desired,the mirror 5 may be placed in advance of the'lens 4 instead of followingit. Then the light rays will become vbroken .into' horizontal directionbefore they` 'strike the lens, with the result that the lattercan beplaced at any point along the horizontal I path of the raysl.Consequently, the distance q can be increased to any desirabledimensions,

with' resultant beneficial eiectupon the permissible value of F andconsequent permissible width of W.. This mode lof disposition istherethen interposed between the focussing lens and the projectionscreen.

`A-diiferent mode of increasing the size o'f the writing plane isillustrated in Fig. 2.

In this modification, the lens 2 is notan ordi-v nary condensing lens,but what is known as an annular Fresnel lens. An enlarged section ofsuch a lens is shown in Fig. 5. The ribs on the under surface' of thislens run circularly around its center. The details of such a lens arewell known, and need no detailed description. See, for instance,Encyclopedia Britannica, 14th ed., vol. 14, page 91. IIt is to beremembered, however, that' because ofthe thick portions of thewriting-panel lens tend to displace'the image somewhat, and because thethicknessof a Fresnel lens changes abruptly at its-ribs, such a lens isapt to project upon the screen a series of concentric rainbow-coloredrings, disrupting the continuity of the image. This undesirable eifectcan be'weakened by using monochromatic light and by increasing thenumber of ribs, thereby decreasing their height differences at thepoints of abrupt change. In any event, the utility vof this particulardesign is limited, and it is therefore not a preferred embodiment.

rn Fig. 3 I have shown stili aA third moda' iication of my invention, inwhich a doubleconvex lens is used in conjunction with' an ordi'- naryplane plate of glass to give the combined functions of the plano-convexlensof Fig. l., The double-convex lens is placed underneath the Awriting plate, and as close to it as possible, as otherwise it is apt todistort the image somewhat.

'Of course, in lieu of'a double-convex lens, a

plano-convex lens with its at face either up-` wards or downwards, orany other form of con-f densing lens, may be employed.

In -a similar manner any other combination of lensesfprisms. etc., witha plane transparent surface may be employed, provided thecombinationproduces the following two eilects:

(l) provide a flat support for the writing paper to be Written on, andv(2) bend the light rays coming from the illuminating source, into thegeneral direction of the focussing lens 4.

It will be clear that in lieu of the simple focussing lens 4, acombination of 2 or '3 lenses in series may be employed, provided theseare placed close to each other so as to effect altogether but oneinversion of the image. Such compound lenses are quite common in theartof projection, and are designed to eliminate spherical and chromaticaberration. For all purposes of this invention such compound lenses areto be regarded'as the equivalent of the lens 4 (or lens L) shown in thevarious figures and diagrams.

, Turning now to Figs. 1'1 to 15 inclusive, these g'ures show severalmodii'lcati'ons of my invention whereby it is' possibleto increase thesize of y trimmed down to straight lines, as shown inde# tail in Fig. 4,in order that each may ilt 'inv l 5' properly with the other to form acontinuous transparent plane. This is explained further .by Fig. 15,which constitutes a section through the writing plane of Fig. 11, both'along' the line' X-X, and along the line Y--Y.

In such an embodiment, fourfocusslng lenses are required in lieu of thesingle lens 4.of Fig. l,

each of these focussing lenses 45, 45 cooperating with a correspondingcondensing lens from -the group 25, 25. Now, if a plane mirror wereemployed `with this modification of my invention, the projection on thescreen. 3 would -be scrambled, because the writing from quadrant Awould'be projected into quadrant D, that from B into quadrant C, etc. Toprevent such dis-l astrous results, the plane mirror 5 of Fig. 1 isreplaced in Fig; 11 by a system of. four mirrors 55, 55, which is shownmore clearly in Fig. 14. The four mirrors are supported on a singleplate 5l, which issupported in inclined position in the same manner asmirror 5 in Fig. l so as to direct the shadow rays in the generaldirection of the screen 3. Buteach mirror on this support vis lifted onits outer corner, for instance by the aidJ of adjusting screws 552 56,and tilted inwardly toward the center of `the support.' TheI tilt ofeach mirror is adjusted-sumciently to cause the shadow rays fromquadrants A, B, C and D of the writing plane to be projected,respectively, into the correctquadrants A, B, C and Dof the screen 3,and to cause these quadrants of projection Vto join up correctly at theedges with'y out overlapping.

In this modification, the light source I5 is likewise replaced by aixture'of .four individual light sources |55, |55, each cooperating withone of the qua writing plan and each provided with a reflector orcondensing lens (not shown) designed to dil regt the ,light toward thecenter of the corresponding quadrant. A

The paths of the rays of light and their cross reflection by the mirrorsystem can be traced in Fig. l1 by following the central light rays a,b, c and d.

Apart from the features' above described, the modification f Fig. 11follows in general the design of Fig. l, and mayhave all the details orvariations of the latter. For instance, the projection screen may beinclined as in Fig. 1. A Also," the mirrors 55 need not be supported onthe. same plate', but may be supported each with its respective lens 45on anl individual support, provided each mirror is placed at the correctangle to reect the light into the corresponding quadrant of theprojection screen. The mirrors may also precede the focussing lenses 45,45, in which event the latter Iare placed substantially vertically,along the horizontal portion of the path of the rays of light, asalready explained in connection with Fig. 1. y

Fig. 12 is a modication of the embodiment shown in Fig. ll, in respectto the mirror system. Instead of four tilted mirrors, a plane mirror 5is here employed, placed at an angle just as in Fig. 1. But after beingreflected from the mirror..

ants A, B, C and D of the composite the light rays are retracted bymeans of a y system of prisms 9 comprising four prisms 95, 95. Eachofthese uprisms is bound by plane faces and tapers from the center ofthe system 9l toward one of its corners. llnqthis fashion, the prismsrefract the light rays a, b, c, d and direct them toward the quadrantsA, B, C, D in correct order. Y A

Fig. i3 is a further modification of the relfleeting system of Fig. 11.Here, the mirror consists of two plane mirrors 56, 56, arranged intrough-like fashion on the common support l,

with the axis of the trough running transversely of the writing desk.Cooperating with this mirror is a prism system 9 consisting of twowedgeshaped prisms 96, 9B placed back to back, with their thin edges ina vertical direction. The action of this compound system on the lightrays can be followed readily by tracing the lines a, b, c, d in thisfigure.

The modification of Figs. 11, 12 and 13 is adapted also -to be used witha writing panel which is a flat sheet of glass, as in my parentapplicationinstead of a system. of condensing lenses. This modification,however, requires the lamps |55 to be spaced farther apart, so that eachlamp is in line with the center of the corresponding quadrant on thewriting plane and the corresponding focussing lens 45. Also, thismodification, requires strong illumination coming from a source of widearea. It is therefore not a preferred modification.

It will be understood that the specific embodiments described above aremerely illustrative and are not intended to limit the invention.' Manyvariations in the details of construction may be made without departingfrom the spirit of this invention.

For instance, the lecture desk may have a floor built in therein, or maybe made without a oor. 'I'he illuminating means maybe a part of thedesk, or may be omitted, to be installed locally I in the auditoriumwith the installation of each unit. The partition 8 in any of themodifications may take any of the forms shown in the drawings of thisapplication, or may be omitted altogether. Likewise, the projectionscreen 3 may be placed inwards of the front wall of the desk, as in Fig.1, flush with it, as in Fig. 3, or may be moved out to the end of thehood I6 or other similar light-excluding extension built onto the desk.f

Instead of a single lens and a single mirror in the optical system, anyAcombination of lenses and mirrors may be employed which, in addition tofocussing and projecting, will eiect one net re` versal of thehorizontal dimension of the lecturers writing and a net zero reversal ofthe top-` to-bottom dimension thereof, thereby orienting the projectedscript on lthe screen in correct manner for reading by the audience. Theprojection screen may be made of fine cloth, ground glass, translucentpaper, or any other material which will transmit shadows through itselfwhile simultaneously scattering somewhat the light passing throughitself. Many other variations liquid. .For instance a large watch glassof spheroid curvature maybe cemented around its periphery to a planesheet of glass to form `a plano-convex shell. A vent provided at asuitable point, communicating with a partially lled, sealed reservoir,may provide an outlet for 4 thermal expansion and contraction of theliquid.

Of course, the liquid for such a purpose should be chosen of highboiling point, say over 80 C.,

.75.' low freezing point, say below 0 C. or below -10 C., and convenientrefractive index, say between 1.33 and 1.75. Water seems to satisfythese conditions to a fair degree. Other con venient liquids may beselected from physical and chemical tables.

1533 and 1535.

As for the sheet of paper 6 which is stretched See, for instance,Handbook of Chemistry and Physics, 19th edition, pages 1526- over thewriting panel 2, I find that the more transparent it is the better thecontrast between writing and background on the projection screen. Bestresults are obtained with highly transparent artificial cellulosicsheeting, for instance nonmoistureproof y ,Cellophane".

I claim as my invention:

l. An apparatus for projecting handwriting, comprising in a single unitof furniture a substantially horizontal writing panel adapted to supporta sheet of light-transmitting writing mal terial on the upper surfacethereof, a substantially vertical projection screen, and a focussingsystem; said writing panel being composed of light-transmitting materialand being provided on the underside and adjacent thereto with lightvcondensing means, thereby being adapted to con- 'dense and to passthrough itself into the space beneath itself light rays coming from asource of illumination above said writing panel through said writingmaterial; said focussing system being disposed between said writingpanel and said projection screen and being adapted to project upon thelatter the light rays which come from the former, and to focus upon saidscreen shadows of any writing or figures done on said writing materialwith a light-absorbing wnfing vmedium; and said screen being composed ofshadow transmitting 'material whereby to transmit said projected lightand focussed shadows through itself to the other face thereof, therebydisplaying the writing or figures as shadows upon an illuminatedbackground; said writing panel, focussing system and projection screenbeing held together in their proper relative positions` by ture unitbeing free of any obstructions or rnembers in the line of sight betweenthe lecturer and his audience.

2. An apparatus for projecting handwriting. comprising in al single unitof furniture asubstantially horizontal writing panel supporting a.

sheet of light-transmitting writing material stretched over the upper,surface thereof, a airbstantially vertical projection screen, andafocussing system; said writing panel consisting of a plano-convex lensof transparent material, therei by being adapted to condense and passthrough itself into the space beneath itself light rays com; ing throughsaid writing material from a source of illumination above said writingpanel; said focussing system comprising'a focussing lens and at leastone plane mirror cooperating with each other to project upontheprojection screen the light rays coming through the writing panelwhile simultaneously focussing uponsaid screen shading material by theaid cf a nent-wending writing medium; and said screen being composed ofshadow transmitting material, whereby to transmit said projected lightand focussed shadows to the other face thereof, thereby displaying thewriting or figures as shadows upon an illuminated background, saidwriting panel, focussing system and projection screen being heldtogether in their proper relative positions by said single unit offurniture, and the latter being shaped and proportioned to servesimultaneously as a lecture desk, and to formV a substantially inclosedspace behind said projection screen,.thereby excluding stray light fromthe inner face of the projection screen; said plano-convex writing panelbeing disposed in an opening in the top of said furniture unit with itsplane surface uppermost, in convenient positionvfor writing thereon by alecturer during the course of his lecture while facing the audience; andsaid projection screen being disposed in said furniture unit with itsouter face displayed to the audience while the latter is facing thelecturer.

3. An apparatus as in claim 2 in combination with means of illuminationlocated' outside said furniture unit above said writing panel, saidilluminating means comprising a source of light and a condensing lens,whereby to throw upon said Writing panel a concentrated beam of light.

4. A combination as in claim 1, the writing panel being formed'by aplane sheet of glass associated with a converging'lens locatedimmediately beneath the same within the interior of the cabinet, wherebyto converge the light passing through said writing panel into theinterior of the cabinet.

5. A lecture desk comprising in a combination a cabinet, a writing panelin the top of the cabinet consisting of light-transmitting material, ashadow receiving screen in one of the vertical sides of the cabinetcomposed of shadow transmitting material, an optical system within thecabinet adapted to intercept shadows coming from the writing panel andto project them upon the screen, said optical system being composed of aplurality of focussing lenses, each of which is adapted to receive lightrays and shadows from a separate region in the writing panel, and eachbeing associated with independently adjustable means for breaking thelight rays passing through said focussing lens and for directing themupon a 'definite and separate region on the projection screen,corresponding in orientation to the region on the writing panel wheresuch rays originated, the said separate regions and orientations beingso selected with respect to each other that the separate projections bythe various focussing lenses upon said screen add up on said screen andpresent to view an orderly and continuous projection of the entire eldof writing laid out on the writing panel.

6. A lecture desk as in claim 5, the writing panel being composed of aplurality of planoconvex lenses tted edge to edge to form by their planesurfaces a continuous writing plane, each ofsaid plano-convex lensesacting to condense light rays passing through itself and to direct ittoward the corresponding focussing lens cooperating therewith.

7. An article of lecture-room furniture comprising a desk having a topmember and wall members of opaque material forming a substantiallyenclosed, darkened chamber; said top' member having at least a portionthereof removed and replaced by a writing panelv composed of lighttransmitting material; and one of the Wall members having likewise aportion thereof `removed and covered up by a shadow transmitting screenpositioned to display to an audience facing said desk light-and shadowsimpinging upon said screen from within said chamber; said writing panelbeing adapted to cooperate with means of illumination external to thedesk whereby to transmit into the interior of the darkened chamber lightcoming from'said illumination means and to transmit shadows of figureswritten on the writing plane with light-absorbing writing material; and'an optical projection system within the interior of said desk placedbelow the light rays coming from said writing panel, said opticalprojection system including imageforming means and image-reversing meanscooperating with each other to focus upon the inner side of theprojection screen an image of shadows coming down from the writing paneland to reverse such image in the horizontal dimension, whereby suchimage when transmitted through the screen material to the outer surfacethereof shall appear to an audience facing the projection screen to havethe same up-and-down and right-and-left orientation as the originalfigures on the writing panel have from the viewpoint of the lecturer.

8. An article of lecture-room furniture as in claimh7, having furthermeans, including a lightconverging lens, associated with said writingpanel whereby to concentrate the light-rays coming through the writingpanel and to bring them substantially to a focus at a region within theinterior of said chamber in the vicinity of said optical projectionsystem.

9. An article of lecture-room furniture as in claim 7, said writingpanel consisting of a planoconvex lens having its plane surfacesubstantially flush with the top surface of said desk, thereby to serveasa writing plane. n

10. An article of lecture-room furniture as in claim 7, having furthermeans associated therewith wherebyA to stretch a sheet oflight-transmitting paper over said writing panel, thereby to serve aswriting paper.

11. An article of lecture-room furniture as in claim 7, said writingpanel consisting of a planoconvex lens having its piane surfacesubstantially flush with the top surface of said desk thereby to serveas a writing plane, and said desk having further meansI associatedtherewith whereby to stretch a sheet of transparent-paper over saidplane surface ofsaid writing panel said writing panel in substantialalignment with DAVID 70

